I’m pretty sure (I’ll explain why) that the problem doesn’t come from a damage within the modules, but is a connection problem from the module and the mainboard.
I started noticing mic problems like you did when using sleep recordings. And only realized that my mic sometimes doesn’t work completely when listening to my own telegram voice messages.
I didn’t wanted to replace the module just yet because I’m a “cheap swabian” and realized I still could get the mic to work clearly by squeezing my fingers in the bottom module area, where the fairphone text is located. Just simply pressing the fairphone sandwich together fixed it for me from about november 2017 until now.
I also wasn’t ready to pay for a module which maybe wouldn’t fix the intermittent contact between the module and the mainboard either. A coworker had her bottom module replaced, because she couldn’t charge her battery anylonger via the USB port. So I wanted to test whether her old module, which suposedly had a working mic, would work in my FP2… but it did not.
So I figured that the problem would either be in my mainboard or an intermittent contact.
I ignored the problem and always had my bluetooth headphones ready to make phonecalls, and if I get called without the headphones I used the speakerphone.
Until now. Now my USB port has an intermittent contact as well, and my fix is to squeeze down on the phone like I did to get the mic to work, so I figure it is the same intermittent contact.
Right now I’m charging my spare battery in a charging station and swap the batteries.
Fun fact: My bottom module I completely decommisioned, I pulled it out of my phone. The mic and USB port didn’t work anyways. I don’t need the vibration motor because I’m redirecting my notifications to my miband (fitness tracker with vibration).
And when I’m taking a call without my headset I’m switching from earpeace to speakerphone mic in a push-to-talk fashion.
I’m considering sending my “defective” module to the support for inspection. Maybe it is fully intact. I hope they are coming up with a new one with usb-C that one I would buy.
In the meantime I’m considering redirecting the second usb port (case upgrade port) to a do it yourself USB port.